In a conventional art, first, an input image is generated by reading dotted contents of printed matter into a computer, as a multi-valued image. Then, printing dots, which are produced by printing, and interference fringes (moire patterns) are removed from the input image by a smoothing process. Thereafter, a contour emphasizing process is performed to recover contours for which level has been lowered by the smoothing process. Finally, a smooth multi-valued image (also called a multi-level image) is obtained.
In such a conventional process, however, when a high-level smoothing process is performed to remove as many as possible of the printing dots or moire patterns, the contour information for a portion of the input image having an originally low contour level is completely lost, so that the contour of that portion can not be recovered by the contour emphasizing process.